Many types of nickel positive electrode exist which can be used in an alkaline electrolyte storage cell:
a pocket-type electrode, obtained by compressing electrochemically active material, in this case hydroxide Ni(OH).sub.2, mixed with a conductor, into a metal pocket having perforated walls so that the electrolyte can impregnate the active material, but the active material cannot escape from the pocket; and PA1 a sintered-type electrode, in which the electro-chemically active material is deposited in a sintered support by chemical or electrochemical precipitation.
In response to the growing needs of users, pasted nickel positive electrodes can now be made. Compared with other types of electrode, a pasted-type electrode contains a larger quantity of electrochemically active material; its capacity per unit volume is therefore increased and its manufacturing cost is lower.
A nickel electrode of the pasted type is made by depositing a paste either on a two-dimensional conductive support such as expanded metal, a grid, a fabric, a solid strip, or a perforated strip, or else in a three-dimensional conductive support that is porous such as a felt, a metal foam, or a carbon foam. During manufacture of the electrode, a volatile solvent is added to the paste to adjust its viscosity in order to make said paste easy to form. Once the paste has been deposited on or in the support, the assembly is compressed and dried in order to obtain an electrode of desired density and thickness.
The main constituents of the paste include the said "active" material, normally in powder form, a polymer binder, and usually a conductor. In a nickel electrode of the pasted type, the active material constituting the paste generally contains a nickel-based hydroxide. Nickel hydroxide is a compound of low conductivity which requires a conductor to be added thereto to enable good electrical percolation.
Document FR-2 567 326 proposes adding nickel powder as the conductor. According to document JP-63 004 561, short nickel fibers can also be used as the conductor for improving conductivity.
Document EP-0 581 275 describes a nickel electrode of the pasted type which contains nickel hydroxide powder as its active material, and fibrous nickel powder as its conductor, the fibrous nickel powder comprising numerous chain-structured zones pointing in all three directions, the average diameter of the chain-structured zones being no greater than 1.3 microns. The nickel powder is advantageously in the form of a mixture of fine particles (of diameter lying in the range 0.6 microns to 1.0 microns) and larger particles (of diameter less than 1.3 microns). The electrode has improved capacity and extended life time.
Document EP-0 658 948 describes an Ni--MH alkaline storage cell provided with a positive electrode of the pasted nickel electrode type constituted by nickel hydroxide, as its active material, and by graphite, as its conductor. The nickel positive electrode provides the storage cell with increased stability at high temperatures of use. Furthermore, it can be seen that the nickel positive electrode cannot be associated with a cadmium negative electrode, because, in that case, the graphite oxidizes into carbonate ions which pass into the electrolyte.
To increase the rapid discharge performance of a non-sintered nickel electrode, document JP-57 138 776 proposes a conductor constituted by a mixture of particles, preferably of graphite powder, and of fibers made of carbon or of stainless steel, for example.
The distribution of the conductive lattice is improved by using, as the conductor, a cobalt compound such as metallic cobalt Co, a cobalt hydroxide Co(OH).sub.2 (JP-6 251 157), a cobalt salt (EP-0 490 991), and/or a cobalt oxide CoO (JP-6 283 170).
During the first charge of an alkaline storage cell provided with a nickel electrode containing a cobalt compound as its conductor, said compound is oxidized into cobalt oxyhydroxide CoOOH in which the cobalt is brought to oxidation number +3. The cobalt oxyhydroxide is stable in the normal operating range of the nickel positive electrode and is insoluble in the alkaline electrolyte. It provides electrical percolation for the electrode.
When stored in the completely discharged state, an Ni--Cd or Ni--MH alkaline storage cell having a nickel positive electrode of the pasted type containing a cobalt compound sees its voltage decrease over time. With increasing duration of storage, its voltage tends towards 0 V, with the reaction kinetics of that phenomenon being a function of storage temperature. In those storage conditions, alkaline storage cells having positive electrodes of the pasted type suffer irreversible loss of capacity due to their positive electrodes.
When the voltage of a positive electrode is close to 0 Volts, the cobalt oxyhydroxide contained therein, which is stable only in the normal operating range of the electrode, slowly reduces. The cobalt is thus brought firstly to oxidation number +2.66 in Co.sub.3 O.sub.4, then to oxidation number +2 in Co(OH).sub.2, and finally to oxidation number 0 in Co. Unfortunately, cobalt hydroxide Co(OH).sub.2 is a compound which is very soluble in electrolyte. consequently, after a storage period of several months, a loss of conductivity can be observed due to the percolation lattice of the pasted electrode partially dissolving. This results in irreversible loss of capacity which can exceed 15%. This irreversible loss of capacity occurs whatever type of cobalt compound is initially introduced into the paste.
Document EP-0 634 804 describes a nickel electrode of the pasted type for an alkaline storage cell having an active material essentially constituted by nickel hydroxide, and having a conductor containing at least one compound selected from cobalt, cobalt hydroxide, and a carbon powder. The nickel electrode of EP-0 634 804 has high energy density over a range of temperatures of use that is larger than that of conventional electrodes.